How can I remove a linked library with Python?
Quite an old thread, but it seems there is finally a solution available.
Newer versions of Blender provide the batch_remove method in the bpy.types.BlendData type.
So you can call:
bpy.data.batch_remove(ids=(my_library,))

my_library is a bpy.types.Library object in this case and no string/name.
Keep in mind that to this date, the documentation states, that this method is considered experimental, but as long as you don't deal with blend file critical data blocks like scenes, it seems you should be fine.